1. Field of the Invention
The present invention relates to a novel leveling apparatus for a vehicle headlamp. More particularly, the present invention relates to providing a leveling apparatus unaffected by ambient temperature and ambient humidity.
2. Description of the Related Art
A leveling apparatus is provided with a drive source such as a motor, which may be remotely controlled by a driver or automatically driven. When driving the drive source, a tiltable member for changing the direction of irradiation of the headlamp, such as a reflector supporting a light source or a lamp body containing the light source and the reflector, is tilted.
FIGS. 8 and 9 show an example of a conventional leveling apparatus for a vehicle headlamp.
A leveling apparatus has a case body b in which a motor c is contained. A worm wheel d is disposed within the case body b rotatably but unmovably in the longitudinal direction and a tapped hole e is provided in the worm wheel d. A worm gear f is engaged with the worm wheel d and rotated by the motor c via reduction gears g. When the worm gear f is rotated, the worm wheel d is rotated.
The leveling apparatus has a shaft h axially moved as the worm wheel d rotates. The shaft h is in the form of a spindle having a threaded portion i, a slide support portion j and a spherical body k. The slide support portion j is non-circular in traverse cross section and passed through a non-circular through-hole 1 formed in the front end portion of the case body b, whereby the slide support portion j is movable back and forth though not rotatable.
The threaded portion i of the shaft h is screwed into the tapped hole e of the worm wheel d and when the worm wheel d is rotated, the thread ridges n formed on the threaded portion i of the shaft h are fed axially by the thread ridges m formed inside the tapped hole e of the worm wheel d, so that the shaft h is moved in the axial direction.
The leveling apparatus is supported by a fixing member having fixed relations with a vehicle body and a tiltable member such as a lamp body or the like so that the axial direction of the shaft h substantially conforms to a direction along the longitudinal direction. The spherical body k provided at the front end of the shaft h is coupled to the tiltable member like a reflector or the lamp body.
When the motor c is driven under remote control or automatically as described above, the worm wheel d is rotated and the shaft h is moved back and forth in agreement with the rotational direction of the worm wheel d and then the part coupled to the front (fore) end of the shaft h out of the whole part of the tiltable member is displaced back and forth, whereby the tiltable member is tilted so as to change the direction of irradiation of the headlamp.
A problem with the conventional leveling apparatus a for a vehicle headlamp as described above arises from the fact that because the thread ridges m of the worm wheel d together with the thread ridges n of the shaft h are formed over several pitches, generally over the range of five to eight pitches (see FIG. 9(a)), a change in torque (hereinafter called the required torque) applied to rotating the worm wheel d becomes conspicuous because of a change in temperature as well as humidity.
If two members are screwed together to effect the output transmission, some play is needed between the adjoining thread ridges to assure that the relative movements of one member over the other are rendered smooth. However, a backlash is produced between the two members when the play is too large and the positional accuracy may deteriorate.
In the leveling apparatus a, the play between the thread ridges m of the worm wheel d and the thread ridges n of the shaft h is set at 0.05 mm or less.
Notwithstanding, if the play is set too small with the thread ridges m and n on two sides staying in mesh over several pitches with their dimensions growing greater as the temperature rises, an accumulation of dimensional errors produced therebetween results in tightening the intermeshing of the thread ridges m and n on two sides (see FIG. 9(b)). Furthermore, the load applied to the motor c and the reduction gears g may increase because of an increase in the required torque. This problem appears notable particularly when the worm wheel d and the shaft h are made of resin materials. If the materials of both the worm wheel d and the shaft h are changed because of wear resistance and transmission efficiency, the problem above also may appear conspicuous because of the difference in linear expansion coefficient. If one of the materials has water absorption properties such as nylon in particular, expansion by water absorption will result and the load applied to the motor c may become considerably greater.
If errors in pitch are produced between the thread ridges m and n on two sides, error accumulates as the number of pitches meshing together grows larger. Then, the intermeshing of the thread ridges m and n on two sides is tightened. The problem in this case is that the required torque increases.